1. Field of the invention
This invention relates to a process for forming a semiconductor thin film, particularly to a process for forming a single crystal region on an amorphous insulating substrate. The present invention can be suitably used for, for example, SOI technique.
2. Related Background Art
In the prior art, a single crystal thin film indispensable for formation of a semiconductor device has been formed generally by epitaxial growth from a single crystal substrate.
However, according to this method, no single crystal can be grown on an amorphous substrate, and it is extremely difficult to effect crystal growth of a crystalline material having different lattice constants or coefficient of thermal expansion from those of the single crystal substrate, whereby the substrate material and the kind of the grown film have been limited.
On the other hand, in research and developments in recent years, there have been developments of three-dimensional integrated circuits to effect higher integration and multiple functionalization by lamination of semiconductor devices on a substrate, or switching transistors of liquid crystal picture elements in which solar batteries or devices are arranged in arrays by depositing semiconductor materials on an inexpensive glass substrate. The technique of realizing a structure having a semiconductor thin film of high quality formed on an amorphous insulating substrate (SOI structure) common to these devices is becoming important.
In recent years, to obtain such SOI structure, various research has been done, and the lateral solid phase growth method (L-SPE) which can be practiced at a process temperature of 600.degree. C. or lower is one of the techniques particular interest.
However, to cause solid phase growth to occur with the single crystal of the substrate as the seed crystal, because it is not electrically insulated from the substrate, and also because a polycrystalline region is formed due to random nucleus generation and growth occurring primarily at the SiO.sub.2 --amorphous silicon interface within amorphous silicon remote from the seed crystal on the substrate, the growth distance from the seed crystal is as short as several .mu.m to 10 and several .mu.m. Further there is a problem of defects remaining within the L-SPE layer (many twin crystals, and dislocations). In order to solve these problems and introduce the L-SPE technique into device application, the surface cleaning technique of the substrate before deposition of the amorphous semiconductor layer becomes very important. Recently, there have been practiced (1) heat treatment at 800.degree. C. under ultra-high vacuum (H. Ishiwara, A. Tambe, and S. Furukawa, Appl. Phys. Lett. 48, 773 (1986)), (2) cleaning by Ar sputtering (heat treatment at 680.degree. C.) (K. Kusukawa, M. Moniwa, E. Murakami, M. Miyao, T. Warabisako, and Y. Wade, Extended Abstracts of the 19th Conference on Solid State Devices and Materials, Tokyo, 1987, pp 179-182), (3) etching with Si.sub.2 H.sub.6 /H.sub.2 gas (heat treatment at 850.degree. C.) (Y. Kunii, and Y. Sakakibara, Jpn. J. Appl. Phys. 26, 1816 (1987)), but in view of the lowered temperature process, cleaning by sputtering of (2) may be considered desirable. The sputter cleaning of the semiconductor with argon has been also used as the pre-treatment of low temperature silicon epitaxial growth by CVD. Although argon ions having an energy higher than the sputtering threshold value of the substrate material, i.e. generally more than 100 eV are used, since heat treatment is also used, the radiation damage and other damages caused at the time of sputtering are reduced (W. R. Burger, J. H. Comfort, L. M. Garverick, T. R. Yew, and R. Rief, IEEE ELECTRON DEVICE LETTERS, Vol. EDL-8 pp. 168 (1987)).
At present, even when the cleaning techniques of (1)-(3) are employed, the lateral growth distance is short, and there are many crystal defects in the semiconductor thin film formed, and there are many remaining tasks for device preparation.